20002 because all of it added together and then divided by two gives you 20002.
Most graphing calculators will do weighted averages pretty easily. It is mostly a matter of data entry.
mx = -2
my = 10
(x, y) = (mx, my)/10 = (-0.2, 1)
Y = 4x - 1
// Plug this in for variable y in equation [1]
[1] 20x - 5•(4x-1) = 5
[1] 0 = 0 => Infinitely many solutions
If you give the clues I could help.